When I was an associate at WSGR, a partner advised me: "Bob, if you ever advise a client to sue for patent infringement you better first check if the defendant has its own patents... otherwise, your client may get countersued!"
Today, Facebook countersued Yahoo for infringement of ten patents. I am guessing that Yahoo expected the countersuit, but sued based on an assessment that the Yahoo patents resulting in $290 million of Google stock would do just as well in days leading up to the Facebook's IPO.
If you are interested in more detail regarding the Facebook patents, please see the Wall Street Journal article: Yahoo v. Facebook v. Yahoo: An Early Breakdown
Copyright © 2012 Robert Moll. All rights reserved.